In a CDMA cellular system, because the same carrier frequency is used repeatedly in every cell there is no need for handovers between frequencies within the same system. However, considering a case such as when existing systems are present together, there is a need for handovers between different carrier frequencies. Three points pertaining to detailed cases are described below.
As a first point, in a cell where there is considerable traffic, a separate carrier frequency is used to accommodate the increased number of subscribers, and a handover may be performed between those cells. As a second point, when an umbrella cell constitution is used, different frequencies are allocated to large and small cells, and handovers are performed between the cells. Then, as a third point, there are cases of handovers between a third generation system, such as a W (Wideband)-CDMA system, and a second-generation system, such as a current mobile telephone system.
When performing handovers in cases such as those mentioned above, it is necessary to detect the power of carriers at the different frequencies. To achieve this detection, the receiver need only have a structure capable of detecting two frequencies. However, this increases the size of the constitution of the receiver, or makes the constitution complicated.
Furthermore, two types of handover method may be considered: a mobile assisted handover (MAHO) and a network assisted handover (NAHO). Comparing the MAHO and NAHO methods, NAHO reduces the burden of the mobile device. However, it is necessary to synchronize the mobile device and the base station, whereby the constitution of the base station and the network becomes complicated and large in order to be capable of tracking each dedicated mobile device.
For such reasons, the realization of the MAHO method is more desirable, but to determine whether or not to handover, it is necessary to measure the strength of carriers of different frequencies at the mobile devices. However, a CDMA cellular system differs from a time division multiple access (TDMA) system used in a second generation, in that it uses ordinarily continuous transmission for both transmission/reception. In this continuous transmission/reception technique, unless receivers corresponding to two frequencies are prepared, it is necessary to stop the timing of the transmission or the reception and measure the other frequency.
There has been disclosed a technique relating to a compressed mode method, for time-compressing the transmission data in the normal mode and transmitting it in a short time, thereby creating some spare time which can be utilized to measure the other frequency carrier. As an example of this, there is Japan Patent Application National Publication (Laid-Open) (JP-A) No. 8-500475 “Non-continuous Transmission for Seamless Handovers in DS-Mobile Radio Communications Systems”. This application discloses a method of realizing a compressed mode, wherein the spreading factor of the spreading code used is lowered to compress the transmission timing.
The method of realizing the compressed mode according to the above application will be explained below. FIG. 13 shows an example of transmissions in a normal mode and a compressed mode in a conventional CDMA system. In FIG. 13, the vertical axis represents transmission rate/transmission power, and the horizontal axis represents time. In the example of FIG. 13, the compressed mode transmission is inserted between normal transmission frames. In the transmission in the compressed mode, a non-transmission period is provided in the downlink frame, and can be set to a desired period of time (duration). This non-transmission period represents idle period during which the strength of the other frequency carrier is measured. In this way, slot transmission can be achieved by inserting the idle period between transmission of compressed mode frames.
In this type of compressed mode transmission, transmission power is increased in accordance with the time ratio between the idle period and the frame (compressed mode frame) transmission duration. Therefore, as shown in FIG. 13, the compressed mode frame is transmitted at a higher transmission power than the frame in normal transmission. Consequently, transmission quality can be maintained even in frame transmission in compressed mode.
Usually, between the GSM and GSM, different frequency component (control channel) is observed by using one observation period (no-transmission period) assigned in every one superframe. However, when a mobile radio communication system in which the UMTS and GSM systems coexist is considered, it requires operation for observing the frequency components between different systems, that is, from UMTS to GSM system. In this case, too, same as in the case of observation between GSM and GSM, an idle period for observing the frequency component of GSM is set in the superframe of the UMTS.
That is, for one frame of superframe in the UMTS, it is necessary to assign the observation period composed of the same number of idle slots as in the case of GSM-GSM observation. However, in the existing technology, due to restrictions in the error correction code and spreading factor for frame transmission, it is difficult to insert all observation period in one frame, and there are many other problems. Therefore, a technology for observing the frequency component of GSM system from the UMTS is expected in the future.
It is an object of the present invention to solve the problems mentioned above by providing a mobile radio communication system, communication apparatus applied in mobile radio communication system, and mobile radio communication method, capable of observing securely the frequency component of an another system from the UMTS even when the UMTS and the another system coexist, and suppressing deterioration of interleaving performance of compressed mode frame in such a case.